Surface the backend's actual error message instead of a generic fallback

The API already returns specific, honest messages (e.g. 'this course
isn't open for signups yet' for an inactive course) — the frontend was
discarding them and always showing a generic 'something went wrong'.

layouts/courses/single.html

@@ -32,10 +32,16 @@
         headers: { "Content-Type": "application/json" },
         body: JSON.stringify({ email: form.email.value })
       }).then(function (res) {
-        if (!res.ok) throw new Error("request failed");
-        form.reset();
-        form.hidden = true;
-        status.textContent = "Check your inbox — day one is on its way.";
+        if (res.ok) {
+          form.reset();
+          form.hidden = true;
+          status.textContent = "Check your inbox — day one is on its way.";
+          return;
+        }
+        return res.text().then(function (msg) {
+          submitBtn.disabled = false;
+          status.textContent = msg || "Something went wrong. Try again in a moment.";
+        });
       }).catch(function () {
         submitBtn.disabled = false;
         status.textContent = "Something went wrong. Try again in a moment.";